Theoretical Study On Optimal Field-free Molecular Orientation By Controlling Low-lying Rotational States

Molecular rotation has quantized angular momentum,the corresponding rotational quantum states occupy the lower energy part of the energy spectrum,and the transition follows the selection rules.Quantum control of molecular rotation to generate field-free alignment and orientation are the fundamental physical problem for exploring the light-matter interaction with potential applications in molecular spectroscopy,stereochemical reactions,quantum encoding,and quantum simulation.This dissertation mainly studies how to design terahertz pulses to achieve coherent control of low-lying rotational states for generating locally optimal field-free molecular orientation and explores the underlying quantum coherent control mechanism.To this end,the thesis firstly studies how to use a single-cycle terahertz pulse to excite the low-lying rotational states of HCN molecules to generate field-free orientation.The research results show that the pure ladder-climbing single-photon excitation process described by the first order term of the Magnus expansion of the unitary evolution operator is dominant in the weak field regime.In the strong field regime,the direct two-photon and multi-photon processes described by the second-and higher-order terms of the Magnus expansion play essential roles.By taking advantage of quantum interference between singlephoton excitation and multi-photon excitation processes,the theoretical maximum orientation value can be obtained by only controlling the field strength of a single-cycle terahertz field that excites the lowest three rotational states.To further study this issue,the thesis derives the amplitude and phase conditions of the lowest three rotational states for generating the maximum field-free orientation.Based on a pure ladder-climbing singlephoton rotational excitation model,the thesis derives the optimal amplitude and phase conditions for terahertz fields,and then designs a quantum coherence control scheme by using a composite THz pulse.The feasibility of this scheme with optimal amplitude and phase conditions is examined in HCN molecules.The research results of this thesis provide an essential theoretical proof for experimentally realizing precise control of low-lying rotational states,leading to locally optimal field-free molecular orientation.